The present invention relates to a method for heat-treating a metal structure applied for making metal crystals considerably coarse, growing a single crystal or making crystals coarse in one direction (unidirectional crystal grain growth) by way of secondary recrystallization as in the case of production of single crystal metals by a strain annealing process or in the case of production of unidirectionally recrystallized metals by a powder metallurgy process.
With a conventional heat treatment for making metal crystals of an alloy considerably coarse, as shown in FIG. 3, a metal or alloy a to be treated is made to gradually pass through a zone-heating portion b defined by heating coils or the like while being exposed to the surrounding atmosphere. A pyrometer is used to sense the temperature of the metal or alloy a or the temperature of the atmosphere in a heating furnace, thereby controlling the electric power of the furnace.
When the metal or alloy a to be treated in the conventional system has large discontinuous portions such as large steps as shown in FIG. 3, heat release from the metal or alloy a varies with time. Even when the temperature of zone-heating portion b is maintained at a predetermined level, the maximum heating temperatures may therefore vary from one point to another on the metal or alloy a, whereby the metal or alloy a may be melted or maintained at temperatures lower than a recrystallization temperature, resulting in failure to obtain metal or alloy products having desired qualities.
In view of the above, the present invention provides a metal heat treatment method capable of effecting a desired and suitable heat treatment of metal or alloy parts having complicated configurations. According to the present invention, a metal or alloy part to be treated is inserted into a mold whose inner surface is formed to receive the part in close spacing therewith whose outer surface is simple and whose cross section perpendicular to the direction of relative movement between the mold and heating means is substantially constant. The metal or alloy part a as well as the mold are simultaneously heated so that the metal or alloy part is heated through the mold so that the maximum heating temperature can be maintained constant at all the surfaces of the metal or alloy part.